Like what you see in the Top 10? The "Buy Her Clothes" button allows you to see what the model was wearing and buy the pieces immediately from that window rather than seeking them out in the Dress-up menu later.
If you already have a piece in your wardrobe, the menu wanrs you that "You already have that!" Similarly, if the item is won from a quest, Seasonal offer, minigame, or level up, you will not be able to buy it directly from the "Buy Her Clothes" window.